Temperature indicating apparatus using oppositely varying capacitors

ABSTRACT

A temperature sensitive transducer element includes a structure having a cavity. Each of a pair of inwardly facing conductive surfaces defining the cavity forms a plate of a different one of a pair of capacitors. A support member disposed in the cavity has a pair of outwardly facing conductive surfaces each forming the opposite plate of a different one of the two capacitors. As the ambient temperature varies, the support member translates relative to the structure along a line between the inwardly facing surfaces so that the capacitance of one of the capacitors increases and the capacitance of the other decreases. Preferably, each capacitor controls the frequency of oscillation of a different oscillator circuit. A mixer circuit responsive to the oscillator circuits produces a square wave the frequency of which is an analog indication of ambient temperature. A counting circuit converts the analog indication into a digital representation.

United States Patent [1 1 Lewis [451 Nov. 20, 1973' [75] Inventor:Howard B. Lewis, La Canada, Calif.

[73] Assignee: Bell & Howell Company,Chicago,

Ill.

[22] Filed: Apr. 18, 1972 [21] Appl. No.: 245,057

[52] US. Cl. 316/247, 317/248, 317/249 R [51] Int. Cl H0lg 7/00 [58]Field of Search 317/247, 248, 249 R,

[56] References Cited UNITED STATES PATENTS 3,460,011 8/1969 Kadlec317/248 X FOREIGN PATENTS OR APPLICATIONS 111,790 11/1940 Australia317/248 620,184 3/1949 Great Britain 317/247 Primary Examiner-E. A.Goldberg Attorney-Robert L. Parker et a1.

[5 7 ABSTRACT A temperature sensitive transducer element includes astructure having a cavity. Each of a pair of inwardly facing conductivesurfaces defining the cavity forms a plate of a different one of a pairof capacitors. A support member disposed in the cavity has a pair ofoutwardly facing conductive surfaces each forming the opposite plate ofa different one of the two capacitors. As the ambient temperaturevaries, the support member translates relative to the structure along aline between the inwardly facing surfaces so that the capacitance of oneof the capacitors increases and the capacitance of the other decreases.Preferably, each capacitor controls the frequency of oscillation of adifferent oscillator circuit. A mixer circuit responsive to theoscillator circuits produces a square wave the frequency of which is ananalog indication of ambient temperature. A counting circuit convertsthe analog indication into a digital representation.

4 Claims, 3 Drawing Figures TEMPERATURE INDICATING APPARATUS USINGOPPOSITELY VARYING CAPACITORS BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to variable-capacitancetermperature transducers. It relates particularly to a circuitarrangement including a pair of capacitors having oppositely directedtemperature coefficients of capacitance and providing a digitalrepresentation of the ambient temperature of the capacitors.

2. Description of the Prior Art Transducers that provide an analogsignal representative of ambient temperature are well known.For example,thermistors, which are resistors made of semiconductive material havinga temperature sensitive resistance, have been used in bridge circuitsand the like for providing temperature indicating analog signals. Asanother example, US. Pat. No. 2,011,710 discloses a transducercomprising a'capacitorhaving a temperature sensitive dielectric materialbetween the plates of the capacitor. Because the dielectric constantvaries as a function of temperature, the capacitance between the platesalso varies.

The performance of temperature transducers can be evaluated in terms ofseveral factors. Deviation from linearity, for example, is a factor tobe considered in evaluating performance. In many uses, it is desired tohave the transducer cause the development of a signal that varies in alinear manner with respect to variations in temperature. Response timeis another factor'to be considered. Generally, it is desired that thesignal developed in a temperature transducer arrangement indicate thepresent temperature; if the transducer is slow to respond to a change intemperature, the signal developed does not accurately indicate thepresent temperature during periods of changing temperature. Anotherfactor relates to tolerances. It is not uncommon to have drasticdifferences between different transducer components made in accordancewith the same specification. For this reason, the design of circuitarrangements that perform in a predictable manner is a difficult taskbecause the tolerances must be taken into account.

Scale factor is another criterion to be evaluated. By scale factor ismeant the percentage of change in a temperature transducer parameter(e.g. capacitance) caused by a specified change in temperature. Too lowa scale factor makes it difficult to distinguish betweentemperature-caused changes and noise. Too high a scale factor, coupledwith the inevitable limit on dynamic range of the transducer parameter,makes it difficult to measure a desired broad range of temperature.

For each of the different arrangements of prior art temperaturetransducers, it will be found that its performance, when evaluated interms of one or more of these factors is not adequate to meetspecifications im posed in many systems.

SUMMARY OFTHE INVENTION An important feature of the present inventionresides in the provision of two capacitors each having a capacitancethat is a function of ambient temperature, wherein the capacitance ofone changes inone direction while the capacitance of the other changesin the opposite direction. v

A temperature transducer element in" accordance with this inventioncomprises a'first and a second conductive surface. Nonconductive meansmount the conductive surfaces to define opposing faces of a cavity. Asupport member is disposed in the cavity and has third and fourthconductive surfaces. The first and third surfaces are opposing and formplates of a first capacitor, and the second and fourth surfaces areopposing and form plates of a second capacitor. A plurality of signalleads are each connected to a different surface for connecting the twocapacitors into an external circuit. Means responsive to variations inambient temperature cause the support member to translate along a linebetween the first and second surfaces so that the capacitance of thefirst capacitor increases and the capacitance of thesecond capacitancedecreases.

In a system embodiment, the invention includes two temperature sensitivecapacitors each exposed to the same ambient temperature. Thecapacitances of the two capacitors vary in opposite directionsresponsive to variations in temperature. Each of the capacitors isconnected in a different one of two variable frequency oscillatorcircuits. Both oscillator circuits are coupled to a circuit thatprovides an indicating signal that varies in accordance with variationsin the difference in the oscillation frequencies of the oscillatorcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of atemperature sensitive transducer element embodying this invention;

FIG. 2 is a block diagram of a temperature sensitive transducer systemembodying this invention; and

FIG. 3 is a schematic diagram of the preferred circuit arrangement usedin an embodiment of this invention.

DETAILED DESCRIPTION FIGS. 1 and 2 show in perspective andschematically, respectively, a temperature sensitive transducer element10. In the orientation shown, rectangular slab portions 12 and 14project downwardly from an upper horizontal slab portion 16. The slabportion 12 is joined to the bottom surface of the horizontal slabportion 16 at its left marginal edge. The slab portion 14 is also joinedto that bottom surface but at a location somewhat to the right of theleft marginal edge. Preferably, slab portions 12, 14, and 16 are eachmade of a material such as quartz so that they have a relatively lowtemperature coefficient of expansion.

A surface 18 of the slab portion 12 and a surface 19 of the slab portion14 are conductive. Preferably, the surfaces 18 and 19 are madeconductive by metalizing the exterior of the quartz material. As shown,the conductive surfaces 18 and 19 are each planar and are parallel toeach other. Slab portion 16 serves as a securing member for joining andsupporting surfaces 18 and 19 in essentially fixed relation to eachother.

The conductive surfaces 18 and 19, mounted as they are by thenon-conductive slabs 12, 14, and 16, define opposing faces of a cavity20 which, in the preferred embodiment, has the shape of an elongatedslot.

A support member 22, disposed in the cavity 20, projects upwardly from alower horizontal slab portion 24. The support member 22 and the slabportion 24 are each made of a material such as ceramic or metal so thatthey have a relatively high temperature coefficient of expansion.

Conductive surfaces25 and 26 of the support member 22 face theconductive surfaces 18 and 19, respectively, to define capacitors C1 andC2, respectively.

The conductive surfaces 25 and 26 can be electrically isolated from eachother if desired so that the two capacitors can be used independently indifferent external circuits. Preferably, however, the conductivesurfaces 25 and 26 are electrically connected together and operate atthe same potential while in use. To effect this electrical connectionthe support member 22 may be a solid metallic piece. Alternatively, whenthe support member is made of ceramic or the like, its entire externalsurface can be metalized. Slab portion 24 serves as a carrier of thesupport member.

A signal lead 27 is electrically connected as by soldering, welding orthe like to the conductive surface 18. A signal lead 28 is electricallyconnected to the surfaces 25 and 26, and a signal lead 29 iselectrically connected to the conductive surface 19.

An upright rectangular support structure 30 supports the right marginaledge of the lower horizontal slab portion 24 and the right marginal edgeof the upper horizontal slab portion 16. Accordingly, these two rightmarginal edges are maintained in relatively fixed spacial relationshipin such manner that slab portions 16 and 24 undergo no significantangular relative motion with changing temperature. On the other hand,the left marginal edge of the lower slab portion 24 is free to movelinearly relative to the upper horizontal slab portion 16. Such relativemovement causes the support member 22 to translate along a line which isperpendicular to the surfaces 18 and 19.

When second order effects such as fringing and the like are ignored, thecapacitance of a parallel place capacitor is given by the product of thedielectric constant of the medium between the plates and the effectivearea of the capacitor divided by the distance between the plates. Thusas the support member translates to the left, the capacitance of C1increases and the capacitance of C2 decreases.

The following brief analysis indicates the effect of variations intemperature on the transducer element 10.

The lengths of the upper and lower horizontal slab portions 24 and 16change by different amounts. The change AL, of the lower horizontal slabportion 24 and the change ALp. of the upper horizontal slab portion 16are given by the equations:

and (1) where 2 T is a reference temperature, a, and an are thetemperature coefficients of expansion of the upper and lower portionsrespectively, and

L is the length of each horizontal portion at the reference temperature(the fact that the lengths are not exactly equal at the referencetemperature is ignored because this has an insignificant effect on theanalysis). Subtracting Equation No. 2 from Equation No. 1 yieldsEquation No. 3

where (3) Ad is the amount of the translation of the support memberrelative to the cavity.

Depending upon the choice of materials, the quantity (a up.) can be ashigh as 12 parts per million (ppm) per degree fahrenheit or can be aslow as 2 ppm per degree fahrenheit. For example, when an aluminum alloymaterial is used in lower slab portion 24 and quartz is used in upperslab portion 16, the differential expansion (or contraction) isrelatively high. As another example, when a material such as quartz isused in the upper portion and Beryllia (BeO) is used in the lowerportion, the differential expansion is relatively low.

In any event, a difierential change in length causes the support memberto translate in the cavity 20. This translation causes the gap betweentwo facing surfaces forming one capacitor to increase and causes the gapbetween two facing surfaces forming the other capacitor to decrease.

The general equation for capacitance of a parallel plate capacitor is:

C e A/d It follows then that Equation #5 6A C' +AC d Ad d G i1 d Ad andAC Ad Ad 2 Ad F0 7F I.) *(Tli and W V 7 "MW AC/C Ad/d the insignificanteffects of higher order terms being ignored because Ad is small comparedto (1,.

Inspection of Equation No. 6 reveals that the percentage change incapacitance is equal to the percentage change in gap between the plates.Accordingly, it is a relatively easy task to select design dimensions toachieve a particular desired scale factor for the transducer element.

In FIG. 2, an oscillator 35 is shown in block diagram form. Thefrequency of oscillation f of oscillator 35 is controlled by thecapacitance C1. Another oscillator 36 has a frequency of oscillation fthat is controlled by the capacitance C2. A mixer circuit 40 is coupledto both oscillator circuits and produces at 42 a square wave outputhaving a frequency equal to the difference between f and f Preferably,the frequency of oscillation of one of the oscillators, say oscillator35, is always higher than the frequency of oscillation of the otheroscillator. This avoids an ambiguity which would otherwise result.

Counting means 45 responds to the output of the mixer and provides adigital indication of the square wave frequency. Such counting means arewell known and the specific construction thereof is not a part of thisinvention. Many instrument manufacturing companies such asHewlett-Packard sell electronic counting instruments which are suitablefor the arrangement of FIG. 2.

A presently preferred arrangement for the circuitry of the twooscillators 35 and 36 and the mixer circuit 40 is shown in FIG. 3.

Oscillators 35 and 36 and mixer 40 are each shown within a dashed block.A conventional power supply 47 has a ground Volt) output and a +Voutput. The ground output is connected by a signal lead 49 tooscillators 35 and 36 and to the mixer 40. The +V output is connected bya signal lead (not shown) to provide DC power to the oscillators and themixer.

In the oscillator 35, an inductor 50 has one end connected to the signallead 27 which is connected to the conductive surface 18. The signal lead28 which is connected to the conductive surfaces 25 and 26 is connectedto ground by the signal lead 49.

In the oscillator 36, an inductor 52 has one end connected to the signallead 29 which is connected to the conductive surface 19.

Each of the oscillators is of the type commonly called Clapposcillators. Since the two oscillators are in most respects identicalonly one (oscillator 35) is described in detail herein.

A transistor 54 has its collector electrode connected to one end of theinductor 50. A separate collector load circuit comprising a seriesconnected resistor 56 and choke 58 couples the collector electrode to+V. Transistor 54 is biased into conduction in a conventional manner. Tothis end, a resistor divider network comprising resistors 60 and 62 anddiode 64 is connected in series between +V and ground, and the baseelectrode of the transistor is connected to the junction of theresistors in the divider network. The diode 54 provides temperaturecompensation for temperature caused variations in the base-emitterjunction voltage. A capacitor 66 is connected between the base electrodeand ground and provides a very low impedance path at the frequency ofoscillation. In this arrangement, transistor 54 operates in what iscommonly called a common base configuration.

A resistor 68 is connected between the emitter electrode and ground toprovide DC feedback to stabilize the DC operating point of transistor54.

The circuit elements that control the frequency of oscillation ofoscillator 35 are the inductor 50, the capacitor C1, a capacitor 70, anda capacitor 72. The inductor 50 and the capacitor C l are connected toform a series circuit between the collector electrode and ground. Thecapacitors 70 and 72 are connected in series with respect to each otherand in parallel with respect to the series circuit.

The design of the oscillator 35 is such that the capacitance values ofeach of capacitors 70 and 72 is much larger than the maximum capacitanceof the variable capacitance of Cl. By way of example, capacitors 72 and74 may each be 360 pf capacitors and the capacitance of Cl may be 33 pfat a reference temperature. With these values, and a value of 0.35 uhfor the inductor 50, the oscillator 35 will have a nominal oscillationfrequency of about 52 megahertz.

The oscillation frequency of oscillator 35 changes from the nominalfrequency approximately in accordance with the following equation:

where (7) f is the nominal oscillation frequency and r is the ratio ofthe change in capacitance of C1 to the nominal value of C1.

Preferably, the value of r varies in a range between 0 and 0.15. By wayof example, if it is desired to measure a temperature range of l,500 F,the value of r can be kept within the preferred range by using Berylliaas the material in the lower portion of the transducer element l0 andusing quartz as the material in the upper portion therof. If it isdesired to measure a temperature range of 25 F, an aluminum alloy can beused in the lower portion thereof.

Inspection of Equation No. 7 reveals that the oscillation frequency is anon-linear function of r. Accordingly, the rate of change in frequencyof an individual one of the oscillators 35 and 36 with respect totemperature is non-linear also. However, when the two oscillationfrequencies are mixed together, the beat frequency obtained is asubstantially linear function of ambient temperature.

The outputs of oscillators 35 and 36 are coupled to the mixer 40 bycapacitors 75 and 76 respectively. In the mixer, the capacitors 75 and76 are each connected to the base electrode of a transistor 80.Conventional tuning networks and a conventional bias network areprovided to cause transistor 80 to produce at its collector electrode anoscillating signal at a frequency equal to the difference between theoscillation frequencies of oscillators 35 and 36. A transistor 81 shapesthis oscillating signal into a square wave for use by the counting means45.

I claim:

1. A variable-capacitance, temperature-indicating element comprising:

a first and a second conductive surface,

non-conductive means mounting the conductive surfaces to define opposingfaces of a cavity;

a support member disposed in the cavity between the conductive surfacesand having third and fourth conductive surfaces spaced from and opposingthe first and second conductive surfaces, respectively, thereby todefine first and second capacitors,

a plurality of signal leads each connected to a different one of theconductive surfaces for separately connecting the first and secondcapacitors into an external operating electrical circuit; and

means responsive to variations in ambient temperature for translatingthe support member along a line between the first and second surfaces sothat the capacitance of the first capacitor increases and thecapacitance of the second capacitor decreases with the capacitance ofeach capacitor changing in an amount sufficient to differentially afiectthe operation of the external circuit thereby to provide an indicationof the ambient temperature.

2. A transducer element according to claim 1 wherein the first andsecond conductive surfaces are each substantially planar and areparallel to each other, and wherein the translating means translates thesupport member along a line perpendicular to the first and secondconductive surfaces.

3. A transducer element according to claim 1 wherein the translatingmeans includes a carrier member having one end joined to the supportmember and an opposite end held in substantially fixed spacialrelationship with the cavity, the carrier member responding having oneend joined to the support member and an opposite end portion held infixed spacial relationship with the other end portion of the securingmember, the securing member having a different temperature coefficientof expansion from the carrier member so that they expand and contract bydifferent amounts responsive to variations in ambient temperature.

mg 6 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,774,089 Dated November 20,1973

Inventor(s) Howard B. Lewis It is certified that error appearsin tbeabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 5, line 66, Equation (7) should contain in the denominator thenumber 1 and not the letter 1.

Column 6, line 58, delete "transducer" and insert-temperature-indicating--;

Column 6, line 64, delete "transducer" and insert--temperature-indicating--;

Column 7, line 4, delete "temperature transducer" and insert--temperature-indicating-. v

Signed and, sealed this 23rd day of April 1971;.

(SEAL) Attest:

IaIDWARD I'LFLETCHEILJR. C. MARSHALL DANN Attesting Officer Commissionerof Patents Disclaimer 3,774,089.H0w0ml B. Lewis, La Canada, Calif.TEMPERATURE INDI- CATING APPARATUS USING OPPOSITELY VARYING CAPACITORS.Patent dated Nov. 20, 1973. Disclaimer filed June 27, 1974, by theassignee, BeZZ c Howell Company. Hereby enters this disclaimer to claims1, 2 and 8 of said patent.

[Oficial Gazette July 1, 1975.]

1. A variable-capacitance, temperature-indicating element comprising: afirst and a second conductive surface, non-conductive means mounting theconductive surfaces to define opposing faces of a cavity; a supportmember disposed in the cavity between the conductive surfaces and havingthird and fourth conductive surfaces spaced from and opposing the firstand second conductive surfaces, respectively, thereby to define firstand second capacitors, a plurality of signal leads each connected to adifferent one of the conductive surfaces for separately connecting thefirst and second capacitors into an external operating electricalcircuit; and means responsive to variations in ambient temperature fortranslating the support member along a line between the first and secondsurfaces so that the capacitance of the first capacitor increases andthe capacitance of the second capacitor decreases with the capacitanceof each capacitor changing in an amount sufficient to differentiallyaffect the operation of the external circuit thereby to provide anindication of the ambient temperature.
 2. A transducer element accordingto claim 1 wherein the first and second conductive surfaces are eachsubstantially planar and are parallel to each other, and wherein thetranslating means translates the support member along a lineperpendicular to the first and second conductive surfaces.
 3. Atransducer element according to claim 1 wherein the translating meansincludes a carrier member having one end joined to the support memberand an opposite end held in substantially fixed spacial relationshipwith the cavity, the carrier member responding to variations in ambienttemperature to change the spacing between its ends, thereby to translatethe support member.
 4. A temperature transducer element according toclaim 1, wherein the non-conductive means includes a securing memberhaving opposite end portions one of which joins the first to the secondconductive surface, and the translating means includes a carrier memberhaving one end joined to the support member and an opposite end portionheld in fixed spacial relationship with the other end portion of thesecuring member, the securing member having a different temperaturecoefficient of expansion from the carrier member so that they expand andcontract by different amounts responsive to variations in ambienttemperature.